Vehicle occupant detection systems are useful in connection with air bags and other pyrotechnically deployed restraints as a means of judging whether, and how forcefully, to deploy the restraint. One fundamental parameter in this regard is the weight of the occupant, as weight may be used as a criterion to distinguish between an adult and an infant or small child.
One prior weight estimation technique is to install an array of variable resistance pressure sensitive elements in the seat, and to sum the individual pressures to determine occupant weight. A restraint system based on this technique is shown and described in the U.S. Pat. No. 5,474,327, issued on Dec. 12, 1995, and assigned to the assignee of the present invention. Another technique, described in U.S. Ser. No. 08/801,218, entitled Adaptive Seating System, filed on Feb. 19, 1997 and assigned to General Motors Corporation, involves measuring and adjusting the pressure in several inflated bladders variously located in a given passenger seat. When a sharp increase in the measured pressures is detected, the passenger weight is estimated based on a comparison of current pressures with previously measured "vacant seat" pressures.
With most of these systems, the weight of the occupant is determined based upon a difference between the measured pressure and a zero, or vacant, seat pressure corresponding to an unoccupied seat. The vacant seat pressure may be determined by a factory calibration procedure, or empirically during normal usage as disclosed in the aforementioned U.S. Ser. No. 08/801,218. However, the vacant seat pressure tends to shift with aging and usage of the seat, in ways that may be difficult to determine empirically. Accordingly, it is desired to provide a system that can reliably and accurately determine the vacant seat pressure.